The present invention relates to integrated circuitry and, more particularly, to affecting the threshold voltage of integrated circuit devices.
Silicon-on-insulator (SOI) technology has been developed for forming transistors and other electrical devices in integrated circuitry. An SOI differs from more common bulk semiconductors in that the active region (the region in which the semiconductor electrical devices are formed) is a semiconductor layer separated from a semiconductor base by a dielectric region. If the semiconductor is silicon as is common, the dielectric typically is silicon dioxide. The active region of the SOI then becomes the layer of silicon which exists between the dielectric (or, in other words, the insulator) and the surface.
Use of SOI technology has the advantage of simplifying isolation of various electrical devices. Moreover, SOI transistors have low parasitic capacitance, low leakage currents, soft error immunity, etc. However, to obtain the full advantage of SOI, it is desirable that if the device is a transistor, it be fully depleting, i.e., that surface depletion extend through the active region to the dielectric region. Full depletion has many advantages, including providing higher mobility for the passage of charge, and it provides integrated circuitry having continuous current/voltage characteristics.
One problem with reducing the thickness of the active region in line with integrating semiconductor devices, is that the doping concentration necessary for threshold control of transistors formed on the active region may prevent full depletion. In this connection, the factor that governs the extent to which the surface depletion level extends into the active region is the concentration of dopants in such region. The high concentration which has been believed to be necessary to provide a desired threshold voltage prevents full depletion when the transistor gate oxide is thin. For example, if the oxide has a thickness of 50 .ANG. units and it is desired that the threshold voltage be maintained at 0.5 V, the doping concentration for such threshold voltage is about 3.times.10.sup.17 /cm.sup.3. With this thickness, a fully depleted device requires for the transistor to be fully depleting an active region thickness of the order of 500 .ANG.. Such a thickness might not be compatible with the optimization of devices as the function of the devices would have a thickness of 500 .ANG. and their resistance will be high. If thick active regions are used, many of the advantages of silicon-on-insulator (SOI) transistors are lost.